1. Field of the Invention
The present invention relates to a photographic optical system using a diffractive optical element (hereinafter referred to as "DOE"), which is applicable to photographic optical systems used in a relatively wide wavelength range, for example, silver halide cameras and electronic cameras. More particularly, the present invention is suitable for use as a photographic optical system adapted for color images formed by development of three or more colors.
2. Description of Related Art
Silver halide cameras, electronic cameras, etc., that are compact and lightweight are favorable for portable use. Accordingly, a large number of schemes for achieving compact photographic optical systems have heretofore been proposed. General photographic optical systems used in cameras, etc. are formed from a refracting system that bends light rays by a refracting action. However, refracting lenses have a radius of curvature to ensure a focal length and to correct aberrations. Therefore, the conventional practice is to increase the thickness of each lens in order to ensure the thickness of the edge thereof and to increase the axial distance between adjacent lenses in order to avoid an interference therebetween. Moreover, because aberration correction is made by combining together positive and negative lenses, the number of constituent lens elements undesirably increases. Accordingly, it has heretofore been difficult to reduce the size of a refracting system to a considerable extent.
In recent years, aspherical surfaces are used to reduce the number of lens elements to thereby attain a reduction in the size of refracting systems. However, aspherical surfaces cannot correct chromatic aberrations, although they can correct monochromatic aberrations, e.g. spherical aberrations and comatic aberrations. Therefore, if the number of lens elements is reduced to a considerable extent, chromatic aberrations are aggravated. Accordingly, there is a limit in achieving a compact refracting system while ensuring the required performance.
Meanwhile, attention has recently been given to diffractive optical elements (DOEs) that bend light rays by a diffracting action. Unlike refracting systems, DOEs have the advantage that the power thereof is independent of the radius of curvature; therefore, a deviation surface can be formed as a flat surface, for example. Moreover, DOEs have a reciprocal dispersion characteristic of -3.45 and hence enable an achromatic system to be realized even by a combination of a positive power and a positive power unlike the conventional refracting systems. In view of such characteristic feature of DOEs, the use of DOEs in photographic optical systems is conceived.
First of all, DOEs will be explained. Regarding DOEs, a detailed explanation is given in "Optics" Vol.22, pp.635-642 and pp.730-737.
Conventional lenses are based on the refracting action at the interface of a medium, whereas DOEs are based on the diffracting action of light. In general, when light enters a diffraction grating as shown in FIG. 1, diffracted light emanating from the diffraction grating satisfies the following relationship: EQU sin.theta.-sin.theta.'=m.lambda./d (a)
where .theta. is the incident angle; .theta.' is the exit angle; .lambda. is the wavelength of light; d is the pitch of the diffraction grating; and m is the order of diffraction.
Accordingly, if the pitch of the ring-shaped diffraction grating is appropriately set, the incident light can be converged on a point. That is, a lens action can be given to the diffraction grating. Assuming that the radius of the J-th grating ring is r.sub.j and the focal length of the diffraction surface is f, if the diffraction grating is arranged such that the optical path difference between a light ray diffracted by the J-th grating and a light ray passing through the center is an integral multiple of the wavelength, the two light rays intensify each other. That is, the following relationship is satisfied: EQU .sqroot. (r.sub.j.sup.2 +f.sup.2)-f=j.lambda. (b-1)
If the focal length is not very long relative to r.sub.j, the grating ring radius r.sub.j may be given by EQU r.sub.j =.sqroot. (2j.lambda.f) (b-2)
Examples of diffraction grating configurations hitherto proposed include an amplitude-modulation type in which a diffraction grating is formed from bright and dark rings, and a phase modulation type in which the refractive index or the optical path length is varied. In the amplitude modulation type DOE, a plurality of orders of diffracted light are generated; therefore, the ratio of the amount of first-order diffracted light to the amount of incident light (hereinafter referred to as "diffraction efficiency") is about 6% at the most. Even if the amplitude modulation type DOE is improved by bleaching, the diffraction efficiency is about 34% at the most. In the phase modulation type DOE, however, the diffraction efficiency can be increased up to 100% if it is formed with a sawtooth sectional configuration such as a that shown in FIG. 2. Such a DOE is known as "kinoform". In this case, the height of the crests of the sawtooth sectional configuration is given by EQU h=m.lambda./(n-1) (c)
where h is the height of the crests, m is the order of diffraction (hereinafter referred to as "design order of diffraction"), and n is the refractive index of an optical member that forms the diffraction surface.
However, because Eq. (c) is an expression of wavelength, the diffraction efficiency 100% can be attained for only one wavelength. In this case, the diffraction efficiency D.sub.M (.lambda.) is given by EQU D.sub.M (.lambda.)=sinc.sup.2 [.lambda.{M-m{(1-n)/(1-n.sub.DOE)}(.lambda..sub.DOE /.lambda.)}](d)
where M is the working order of diffraction; m is the design order of diffraction; .lambda. is the working wavelength; .lambda..sub.DOE is the design wavelength; and n and n.sub.DOE are the refractive indices for the wavelengths .lambda. and .lambda..sub.DOE, respectively, of the optical member forming the diffraction surface.
The above expression represents that the diffraction efficiency is smaller than 100% for a wavelength other than the design wavelength. As the m th-order diffraction efficiency reduces, other orders [e.g. (m+1)th order and (m-1)th order] of light occurs, and if the other orders of light becomes large in quantity, the light may be undesirably detected as flare. In the following description, m th-order light will be referred to as "design-order light", and the other orders of light will be collectively referred to as "unwanted-order light".
An optical element formed by stepwise approximation of the kinoform configuration as shown in FIG. 3 is known as a "binary optical element", which can be produced relatively easily by a lithographic technique. In the case of binary optical elements, it is known that a diffraction efficiency of 81% is obtained by 4-step approximation; 95% by 8-step approximation; and 99% by 16-step approximation.
DOEs, which have the above-described features, have already been used, for example, in pick-up lenses for compact disks and head-up displays (HUDs) that project an image into the driver's visual field sin the forward-facing front window of an automobile, for example. Thus, DOEs are used only in monochromatic optical systems such as pick-up lenses and other optical systems, such as HUDs, in which the working wavelength range is narrowed to the order of 30 nanometers in order to increase visibility.
However, general photographic optical systems use a wavelength range considerably wider than those used in the above-described optical systems. Accordingly, if a DOE is used in a general photographic optical system, flare increases and affects the image quality. Therefore, it is necessary to solve the problem of unwanted-order light.
Accordingly, various methods have been proposed to solve the problem of flare due to unwanted-order light.
In Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 6-194571, a flare quantity in an endoscope objective lens having a DOE is defined by determining a difference between the amount of incident light and the amount of design-order light defined from the diffraction efficiency, and a kinoform blaze wavelength (i.e. a wavelength at which the diffraction efficiency is increased to 100%) is appropriately set, thereby minimizing flare. Further, a weighted flare quantity is defined by taking into consideration the characteristics of the light source and image pick-up device used, and a blaze wavelength is appropriately set to thereby minimize flare.
In JP(A) 7-324262 and 8-43767, a quantity of design-order light is defined in a photographic lens for a camera having a DOE from the diffraction efficiency of the DOE, the spectral characteristics of the image pick-up device and the transmittance of the lens, and a conditional expression is set so that the defined quantity of design-order light is maintained. This means that the amount of flare due to unwanted-order light is consequently minimized. In JP(A) 8-43767 in particular, a wavelength at which the diffraction efficiency is maximized is also set.
In WO95/18393, a quantity of design-order light is defined from the diffraction efficiency in a photographic lens for a camera having a DOE, and a blaze wavelength is set so that the quantity of design-order light is increased.
These inventions employ a method of increasing the efficiency of utilization of design-order light in order to minimize flare due to unwanted-order light.
Hitherto, it has been considered that the problem of flare is minimized in a photographic optical system using a DOE by increasing the efficiency of utilization of design-order light.
However, it has been found that when color photography is carried out with an optical system using a DOE, blue or red flare occurs particularly conspicuously in the color image (such flare will hereinafter be referred to as "color flare").
In the prior art, however, there is almost no mention of color flare, and hence no method or scheme for solving the problem of color flare is disclosed, as stated below.
In JP(A) 6-194571, there is no mention of a specific phenomenon concerning flare nor color of flare. Regarding the spectral characteristics of the image pick-up device, only those for black-and-white images that are expressed by one function are considered. Therefore, the problem of color flare relating to color images remains unsolved in the invention of JP(A) 6-194571.
In JP(A) 7-324262, consideration is also given to only black-and-white images; therefore, the problem of color flare remains unsolved.
In JP(A) 8-43767, there is a slight mention of color flare, but a specific method of solving the problem of color flare is not disclosed.
In WO95/18393, there is no mention of a specific phenomenon concerning flare nor color of flare, and consideration is not given to an image pick-up device. Therefore, the problem of color flare remains unsolved.
Thus, it will be understood that the problem of color flare cannot be solved by any of the prior art devices. Accordingly, it is necessary to note the color flare and to find out a method of minimizing it.